Bipolar Junction Transistors (BJTs) have a long and diverse history of use in the electronics industry. They are used as a switching mechanism and for amplification in a wide range of applications. BJTs are highly sought after in power electronics because of their low turn on/off times, low voltage drop, high power, voltage, and current ratings, as well as their little leakage current when in blocking mode. In power electronics, they are mostly used for switching applications. The biasing conditions of the junctions dictate the three different modes of operation that bipolar junction transistors (BJTs) display: cut-off, active, and saturation.

A semiconductor device with three layers and three terminals that alternate between p-type and n-type layers is called a bipolar junction transistor (BJT). Three terminals in the BJT is the Collector, Base, Emitter. The arrangement of their layers distinguishes NPN and PNP, the two varieties of BJTs. The base of an NPN BJT is built of p-type material, whereas the emitter and collector are made of n-type material. On the other hand, the base of a PNP BJT is constructed of n-type material, while the emitter and collector are composed of p-type material. The collector-base junction (CBJ) and base-emitter junction (BEJ) are the two junctions found in a BJT.

What is BJT Amplifier?

A Bipolar Junction Transistor (BJT) amplifier is a type of electronic amplifier that uses bipolar junction transistors to amplify electrical signals. Bipolar junction transistors are three-layer semiconductor devices that consist of an N-type layer (negatively doped), a P-type layer (positively doped), and another N-type layer.

BJTs are of two types:

• NPN
• PNP

NPN Transistor

In NPN transistor left and right semiconductor are N-type and middle semiconductor is P-type and the direction of the current outside, towards the Emitter as shown in fig.

NPN Transistor

PNP Transistor

In PNP transistor the N-type diode is sandwich between the two P-type semiconductor and the direction of the current is going into the p-type as shown in below fig.

PNP Transistor

Working Principle of BJT Amplifiers

An active region that is biased is the NPN transistor. In this case, the collector-base junction is reverse biased while the base-emitter junction is forward biased. Therefore, as compared to the collector-base junction’s width, the base-emitter junction’s depletion region’s width is less. By lowering the barrier potential and facilitating the flow of current from the emitter to the base, the forward biased BE junction will aid.

NPN transistor bases often have fewer holes than the electrons in the emitter because they are narrow and mildly doped. The base current flows as a result of holes in the base recombining with electrons in the emitter region. Typically, the electrons’ direction

The emitter current is equal to the total of the base current and collector current, as stated by Kirchhoff’s Current Law. In general, the emitter current (I_E) and collector current (I_C) will always be greater than the base current (I_B).

I_E = I_C + I_B

Working of BJT Amplifier Circuit

BJT amplifiers are essential in many different applications. There are three primary zones in which Bipolar Junction Transistors (BJT) can be operated. Saturation, Active, and Cut-off regions are those. BJT must function in the active or linear areas in order to function as an amplifier. We shall employ the appropriate BJT amplifiers in accordance with the need.

Therefore, after learning about and comprehending the operation of BJT amplifiers, it is simple to comprehend the operation of JFET amplifiers and MOSFET amplifiers. This article will go over the various types and functions of the BJT amplifier.

Circuit Diagram

If we considered the circuit input signal is given and then its respected output is out of phase. The signal is amplify in the BJT uses a technique known as current amplification to amplify signals. There are three layers in it: base, collector, and emitter. The base is narrow and mildly doped, the emitter is substantially doped, and the collector is moderately doped. Electrons (or holes in PNP) flow from the emitter to the base of the base-emitter junction when voltage is supplied, causing the junction to become forward-biased. The majority of electrons don’t recombine and proceed to the collector because of the thinness and light doping of the base. Signal amplification occurs when this movement produces an output current at the collector circuit that is greater than the input current at the base circuit.

Amplifier Circuit diagram

In Common emitter configuration (NPN)

According to the kirchhoff Current law.

I_E = I_C + I_B

Input A.C signal phase -> 0 degree.

Output generated signal phase -> 180 degree.

Types of BJT Amplifiers

BJT works in the 3 configuration as mention below :

• Common Emitter (CE) Configuration
• Common Base (CB) Configuration
• Common Collector (CC) Configuration

Common Emitter (CE) Configuration

• The emitter terminal in this setup is shared by the input and output.
• It provides voltage gain and high power gain.
• Commonly used for amplifying weak signals.

Common emitter configuration

In the CE configuration

According to the summing current law,

I_E = I_B + I_C

I_E -> Emitter current
I_B -> Base Current
I_C -> Collector current

Common Base (CB) Configuration

• The base terminal in this setup is shared by the input and output.
• It provides current gain and low input impedance.
• It is often used in high-frequency applications.
  
  

  Common-Base configuration

In the CB configuration

I_E = I_B + I_C

Common Collector (CC) Configuration

• In common collector configuration, the collector terminal is common to both the input and output.
• It provides voltage gain less than unity but has high current gain.
• It is useful for impedance matching between stages of a multi-stage amplifier.

Common-collector configuration

In CC configuration

I_E = I_B + I_C

Characteristics of BJT Amplifier

BJT amplifier working in the three regions which are mentioned below , also Input – output characteristics of the BJT is shown in below figure, where:

A_V -> slope of the curve in the active region

V_O -> Output voltage of respective input voltage.

In cutoff and saturation region BJT act as the switch and in the Active region BJT as the amplifier.

characteristics of BJT amplifier

Active region

In the active region, the signal gets amplified and the phase reverses from 0⁰ (degree) to 180⁰ (degree). This region is in between active and saturation region. Transistor is operate in this region when collector is reverse and emitter is forward biased.

Active region

I_C = I_B. β

I_C -> Collector current.

β -> current amplification factor.

Saturation region

Next region in transistor behave as a close switch. Collector and emitter are sorted. The CE current is maximum in the saturation region.

Saturation region

Here, in this region

I_C = I_E

I_C -> collector current, I_E -> emitter current

Cutoff region

In this region transistor behave as a open switch, hence current between collector, emitter and base is zero.

Cutoff region

Here ,

I_B = I_C = I_E = 0 (No current flow)

I_B -> Base current

I_C -> Collector current

I_E -> Emitter current

Comparison Between Types of BJT Amplifiers

Characteristic	Common base (CB)	Common emitter(CE)	Common collector (CC)
Input Resistance	Very Low (<100 ohm)	Low( < 1K)	Very High(750K)
Output Resistance	Very high	High	Low
Current gain	Less than 1	High	Very high
Voltage gain	Greater than CC and less than CE	High	Low(<1)
Power gain	Medium	High	Medium

BJT Amplifier Formula

In BJT we have CE, CC and CB :

Current Gain

We have Alpha (α) , Beta(β). Alpha (α) is defined as the ratio of the collector current to the base current α = I_C / I_B . Beta(β) is defined as the ratio of the collector current to the Emitter current β = I_C/I_E . We know that,

I_E = I_C + I_B

Relation between α and β, given below

α = I_C / I_B => α . I_B = I_C ,

β = I_C / I_E => β . I_E = I_C ,

α = β / (β + 1)

I_E = I_C / α = I_B.(1 + β)

I_C -> Collector current

I_B -> Base current

I_E -> Emitter current

Current Loss

However a BJT has two P-N junctions and three terminals. When bjt is biased in its active region, the collector base jcn is reverse biased. The collector base junction reverse saturation current is the emitter current in the absence of base current. This is also called leakage current.

DC Analysis of BJT Amplifier Circuits

For the DC analysis of the BJT, we are going to use common-emitter amplifier circuit as shown in below figure.

Thevenin’s theorem and Kirchoff’s voltage law are used to create a dc equivalent circuit after eliminating the coupling and bypass capacitors, the load resistor, and the signal source in the DC analysis of a common-emitter amplifier circuit.

DC analysis Circuit diagram

We know capacitor block the D.C current so for the D.C analysis all the capacitor in the circuit will be open.

Frequency of D.C current is = 0

So X_C = infinite , Open circuit ( that’s why block the dc current)

Then convert the this circuit into the Thevenin equivalent circuit.

Apply Input KVL get the value I_BQ , I_CQ .

Apply output KVL and get the value of the V_ECQ .

After getting all value plot the graph between I_C and V_CE as shown below

DC analysis curve

The intersection point is the Q-point,

B = V_CC

A = V_CC / R_C

Advantages and Disadvantages of BJT Amplifier

The advantages and disadvantages of BJT amplifier are shown below:

Advantage

• The gain bandwidth of the bipolar junction transistor (BJT) is broad.
• High frequency performance is greater with the BJT.
• The voltage gain of the BJT is superior.
• Applications requiring low or high power can use the BJT.
• The current density of the BJT is high.
• The forward voltage drop is minimal.

Disadvantage

• More noise was created by the bipolar junction transistor (BJT).
• Radiation has a greater effect on BJT.
• The thermal stability of BJT is low.
• BJT switches at a low frequency.
• Its base control is extremely intricate. Thus, it needs to be handled carefully and could cause confusion.

Application of BJT amplifier

• BJT used as amplifier in active region.
• BJT used as Switch in Cutoff region, Saturation region.
• Direct current (DC) signals can be produced by rectifying alternating current (AC) signals using BJTs.
• In order to increase the power of audio signals, BJTs are frequently utilized in audio amplifiers.
• BJTs can be used as current sensors in circuits where it’s critical to monitor and regulate the flow of current.

Conclusion

Amplifiers are a great tool for boosting a tiny signal’s voltage, current, or power to make it bigger. When there is a signal, a small signal amplifier only uses a small portion of its load line. The quotient of the input voltage and the output voltage is used to calculate the voltage gain of small-signal amplifiers. Because it offers the highest voltage gain, the common-base small-signal amplifier is the optimum device for increasing voltage. the common-emitter small-signal amplifier has the second-highest voltage gain and exhibits a 180-degree phase difference between the input and the output.

The minimal voltage gain and the absence of phase difference between the input and output signals are found in the common-collector small-signal amplifier. The emitter in a small-signal amplifier can be both an input and an output, while the base and collector only have one purpose—that of being an input and an output, respectively. If and only if the base-collector and base-emitter functions are reversed- and forward-biased, respectively, the transistor functions in the active region, where it functions as an amplifier. In a common-collector small-signal amplifier, the emitter is the output and the base is the input. If collector is common between the base and the emitter then the collector is ground.

FAQs on BJT Amplifier

What do you mean small signal amplification?

The amplifier is referred to as a small signal or voltage amplifier when the input signal is relatively weak and causes less little swings in the output current relative to its quiescent value.

Explain Phase reversal term.

When the input signal voltage rises in a positive direction, the output voltage rises in a negative direction, and vice versa. The input signal and output voltage have a 180-degree phase change as a result of what is known as phase reversal.

Describe the primary uses that a common-collector amplifier can have.

We have three topologies (CE, CC, and CB), the common-collector amplifier has the highest input resistance and the lowest output resistance. The voltage gain is less than unity, the current gain is equal to that of the CE amplifier. As a buffer amplifier between a low load and a high impedance source, this circuit has several uses.

Explain the term Operating point.

The operating point is defined as the zero signal levels of I_C and V_CE. It is known as the operational point because, when a signal is applied, fluctuations in I_C and V_CE occur around this point. It is sometimes referred to as the Q-point or quiescent.

Estimate the difference between the NPN and PNP transistor.

The direction of current flow and voltage polarity are the main areas of difference between an NPN and PNP transistor. When a positive voltage is given to the base of an NPN transistor, current can flow from the collector to the emitter since electrons make up the bulk of the charge carriers in the transistor. On the other hand, when a negative base-emitter voltage is supplied to a PNP transistor, current flows from the emitter to the collector since the bulk of the charge carriers in the transistor are holes. Because PNP transistors require a negative base-emitter voltage to operate, NPN transistors require a positive one, which has an impact on their biasing as well.